When one or more rolls of a road roller, especially one for compacting asphaltic road material, is or are also steerable, it is desirable that each roll be split into two or more roll shells which are rotable relative to each other. This is because otherwise scuffing of a newly laid asphalt mat will likely occur, the tighter the turn the greater the likelihood of scuffing. At the same time, modern asphalt paving practice has turned more and more toward the use of vibratory rolls in order to increase the density and uniformity of the mat and yet reduce the weight of the roller and the time required to do so.
But split rolls which can also be vibrated are not found in the prior art without many complications and deficiencies. The root of these lies in the substantial extra weight inherent in known vibratory split roll designs compared with non-vibratory split rolls. This is true not only when the vibratory mechanism is located outside the roll itself, and acts upon the overall roll assembly, as in U.S. Pat. No. 3,595,145, for instance, but especially when the vibratory mechanism is disposed within and largely carried by the roll itself, acting just upon the outer roll shell, as in U.S. Pat. No. 3,605,582, for instance. Apart from whether the vibratory mechanism is within or without the roll, some additional means in the case of split rolls must be employed which not only allows the rolls to vibrate together as a unit relative to the remainder of the roller, but which also allows them to rotate relative to each other and yet maintains them together in rigid axial alignment. One approach in the prior art has been to use an additional center frame member carrying the adjacent ends of a pair of heavy shafts upon which roll shells are journaled, thus also leaving a gap between the shells, as in U.S. Pat. No. 3,605,582 referred to. Another approach which has been considered is to use a single, large rigid shaft upon which both roll shells are journaled and associated with which is a vibratory mechanism of the eccentric type.
The trouble with all these approaches, however, is that they very materially increase the weight or mass of the roll. The greater the mass to be vibrated the less the amplitude of the roll for a given applied force at a given frequency. To increase the amplitude a greater applied force is required which in turn means greater eccentricity or heavier eccentric weights unless frequency is sacrificed. Reducing frequency, however, reduces efficiency of the roller because it decreases the speed at which the roller can travel along the new mat. Higher frequencies are desirable because they both increase the vibratory force and permit greater road speeds without impermissible skipping or gaps between the compactive thrusts upon the mat. But increasing the effective eccentric mass or the frequency, that is to say, the applied force, in order to compensate for increased weight of the roll demands larger bearings and more drive power with consequent greater frictional losses and problems of dissipating heat transmitted to the bearing lubricant. Indeed, it is these difficulties, perhaps, which account for the scarce use of vibratory split rolls for steering a roller, despite their desirability. Rather, the much more common practice is to use a relatively small, non-vibrating split roll or "tiller roll" for steering purposes while incorporating the vibratory mechanism in a single large roll which is non-steerable. However, when both rolls must necessarily be steered, as for instance in a tandem double articulated roller of the type shown in U.S. Pat. No. 3,868,194, the problems of also incorporating vibratory mechanism within them become more acute. The present invention arose in that context and while it is particularly designed for that type of roller, it is also applicable to any roller using the split rolls.